Since the 1970s a seemingly paradoxical situation has been noticed across the Sahel region. Decreasing rainfall was met in some zones, such as the Sudano-Guniean zone with decreased surface water and runoff, as would be expected (Gal et al. 2017). However the Sahelian zone saw an increase in river discharge and runoff, this was recorded across the region in multiple studies (Amogu et al 2010; Mahe et al, 2010).
This phenomenon is thought to be linked to the rapid land use change in the Sahel reason associated with agriculture, particularly millet farming, and high levels of firewood collection linked to a rapidly growing population (Favreau et al, 2009; Leblanc et al, 2007). Millet production is estimated to have increased by 600% in Niger since the 1950s (Favreau et al, 2009). This land clearance has had a sizeable impact on the hydrological properties of the area. Firstly the removal of natural woody savannah and its replacement with either bare land or millet reduces rainfall interception (Reifsnyder and Darnhofer, 1989). This is then combined with a decrease in infiltration rate due to soil crusting caused by land clearance leaving soils more exposed to forces such as raindrop energy (Favreau et al, 2009). Decreased permeability of the surface soil combined with less interception means that when there is rainfall there is a greater chance the infiltration capacity will be surpassed, leading to infiltration excess (Hortonian) overland flow. This overland flow then goes on to collect in ponds where it has been found to recharge aquifers (Leblanc et al, 2007). This land use change, particularly the change towards larger expanses of cropland, has been used by many to explain the situation in the Sahel. However this cannot be the sole cause as similar trends are being witnessed in pastoral areas of the Sahel (Gal et al. 2017).
Much of the Sahel region does not rely on crop based agriculture but is instead pastoral communities, however here too the paradox can be seen. Gardelle et al. showed this in their research in Northern Mali where there has been an increase in the area of ponds (Gardelle et al, 2010). This example too seems to be explainable through vegetation change, however not anthropogenic change. It is theorised that past droughts, such as those the Sahel experienced in the 1970s and 80s may have damaged ecosystems to the extent that large volumes of vegetation, particularly those of woody and herbaceous species, are yet to fully recover (Gardelle et al, 2010). As with anthropogenically caused loss of vegetation this has increased Hortonian overland flow by reducing the soils hydraulic conductivity and increasing top soil crusting (Gardelle et al, 2010).
This increased runoff, be it from human or natural causes, is the start of a positive feedback loop. Increased runoff leads to increased erosion and thus the development of a drainage network. Water flowing through this drainage network does not infiltrate into the soil and so does not supply plants with water or nutrients, thus leading to further vegetation death and so more overland flow (Gal et al. 2017).
Whilst the importance of agricultures role in creating the hydrological situation we are seeing in the Sahel is debated it will undoubtedly be greatly effected by it. Firstly it presents a great opportunity to increase crop yield by utilising groundwater. Favreau et al found in their study in Southwestern Niger found that groundwater recharge rates had increased from 2mm a prior to land clearing to 23mm a today (Favreau et al, 2009). The renewable nature of this groundwater means utilising it for irrigation could be a sustainable means of maintaining a reliable harvest in an area that is prone to rainfall variability (Röckstrom and Barron, 2007). As I mentioned in an earlier post such groundwater irrigation may become increasingly important across Africa as climatic changes make rainfall less and less predictable. Utilising this increasing supply of groundwater may not only be useful for farmers but may avoid the issues associated with a rising watertable. If the water table is allowed to rise to the soil surface then evaporation may lead to salination, which can cause health problems and make land less arable, an increase in groundwater fed ponds may also provide breeding grounds for parasites and mosquitos (Favreau et al, 2009).
Until now I've only discussed changing volumes of water, however it is also important to discuss water quality. Land clearance and increased overland flow has increased the leaching of nutrients such as nitrates from the soil, these have then been transferred into the groundwater (Favreau et al, 2009). Whilst this nitrate rich water is damaging to human health it offers great opportunities for agriculture (Townsend et al, 2003). Aside from water nitrates are one of the main limiting factors to millet production in the Sahel (Buerkert and Hiernaux, 1998). Therefore by utilising this nitrate rich water for irrigation it may be possible to increase crop production without the need for costly fertilisers.
However as stated in the Gal et al paper the Sahelian Paradox is not yet fully understood, and thus we can't fully understand how the situation may change if the above suggestions are put in place let alone the effect of climatic changes in the area on this unusual hydrological system. Therefore whilst there seems to be great potential associated with this runoff and associated groundwater more research is needed to fully understand the system before farmers are encouraged to invest in systems to utilise this groundwater. It is also important to remember that whilst there may be some positive outcomes this is not a positive situation. Okechukwu Amogu points out that this is a phenomenon that seems to have been caused by land degradation in the form of processes such as land clearance and soil crusting (Amogu et al 2010). Increased runoff also means reduced lag time and thus there has been a higher occurrence of floods, presenting a risk to both lives and livelihoods in the Sahel region (Amogu et al 2010).
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| The location of the Sahel. Source |
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| A pastoral farmer in Burkina Faso. Source |
This increased runoff, be it from human or natural causes, is the start of a positive feedback loop. Increased runoff leads to increased erosion and thus the development of a drainage network. Water flowing through this drainage network does not infiltrate into the soil and so does not supply plants with water or nutrients, thus leading to further vegetation death and so more overland flow (Gal et al. 2017).
Whilst the importance of agricultures role in creating the hydrological situation we are seeing in the Sahel is debated it will undoubtedly be greatly effected by it. Firstly it presents a great opportunity to increase crop yield by utilising groundwater. Favreau et al found in their study in Southwestern Niger found that groundwater recharge rates had increased from 2mm a prior to land clearing to 23mm a today (Favreau et al, 2009). The renewable nature of this groundwater means utilising it for irrigation could be a sustainable means of maintaining a reliable harvest in an area that is prone to rainfall variability (Röckstrom and Barron, 2007). As I mentioned in an earlier post such groundwater irrigation may become increasingly important across Africa as climatic changes make rainfall less and less predictable. Utilising this increasing supply of groundwater may not only be useful for farmers but may avoid the issues associated with a rising watertable. If the water table is allowed to rise to the soil surface then evaporation may lead to salination, which can cause health problems and make land less arable, an increase in groundwater fed ponds may also provide breeding grounds for parasites and mosquitos (Favreau et al, 2009).
Until now I've only discussed changing volumes of water, however it is also important to discuss water quality. Land clearance and increased overland flow has increased the leaching of nutrients such as nitrates from the soil, these have then been transferred into the groundwater (Favreau et al, 2009). Whilst this nitrate rich water is damaging to human health it offers great opportunities for agriculture (Townsend et al, 2003). Aside from water nitrates are one of the main limiting factors to millet production in the Sahel (Buerkert and Hiernaux, 1998). Therefore by utilising this nitrate rich water for irrigation it may be possible to increase crop production without the need for costly fertilisers.
However as stated in the Gal et al paper the Sahelian Paradox is not yet fully understood, and thus we can't fully understand how the situation may change if the above suggestions are put in place let alone the effect of climatic changes in the area on this unusual hydrological system. Therefore whilst there seems to be great potential associated with this runoff and associated groundwater more research is needed to fully understand the system before farmers are encouraged to invest in systems to utilise this groundwater. It is also important to remember that whilst there may be some positive outcomes this is not a positive situation. Okechukwu Amogu points out that this is a phenomenon that seems to have been caused by land degradation in the form of processes such as land clearance and soil crusting (Amogu et al 2010). Increased runoff also means reduced lag time and thus there has been a higher occurrence of floods, presenting a risk to both lives and livelihoods in the Sahel region (Amogu et al 2010).
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